The present invention relates to fiber optic systems, and more particularly, is directed to a polarization mode dispersion compensator for long fiber cables.
Polarization mode dispersion (PMD) generally refers to variations in the time delay of a polarized optical signal traveling through an optical transmission system, such as a single-mode optical fiber. PMD arises in an optical fiber because of asymmetries in the optical fiber core, such as core ellipticity created during optical manufacturing and bending stresses resulting from handling the fiber or installing the fiber. Asymmetries in the fiber core cause random changes in the state of polarization (SOP) of optical signals propagating through the fiber. Different SOPs propagate through the optical fiber core at different speeds, resulting in pulse distortion in a transmitted optical signal. Additionally, asymmetries in the core are susceptible to environmental changes, such as temperature or fiber movement, which occur rapidly and further distort the transmitted optical signal.
First order PMD refers to a time delay between two orthogonally polarized principal states of polarization (PSPs). The PSPs are a convenient basis set to describe and characterize each SOP and to evaluate the effects of PMD in the fiber. Using the PSPs as a basis set, each SOP propagating through the fiber is represented as a linear combination of the two orthogonally polarized PSPs. The pulse distortion is a function of the varying delay between the PSPs.
Second order PMD refers to the frequency dependence of the first order PSP. Second order PMD further distorts the optical signal propagating in the fiber.
Differential group delay (DGD) is the delay time between the two principal states of polarization in single-mode fiber caused by different propagation group velocities of the modes. The DGD is a function of wavelength and fiber environment and fluctuates in time. Therefore a single DGD measurement cannot characterize a fiber. Instead a PMD measurement is necessary, corresponding to an average or root mean square (rms) value of DGD over some wavelength range, depending on the method.
FIG. 1 shows a feedforward PMD compensator. Transmitter 10 sends a lightwave signal along optical fiber 15 to a beam splitter (not shown) that provides the lightwave signal to PMD detector 2 and to PMD compensator 1. Transmitter 10 includes a light source, such as a laser diode, and optionally includes a polarization scrambler that randomly varies the polarization state of the optical signal prior to providing the optical signal to optical fiber 15. PMD detector 2 calculates the PMD present in the transmitted lightwave, and controls PMD compensator 1 to compensate for the PMD in the transmitted lightwave. PMD compensator 1 compensates the transmitted lightwave, and delivers a compensated lightwave to receiver 90.
Co-pending U.S. patent application Ser. No. 10/338,278, filed Jan. 8, 2003, having common inventors and assignee herewith, presents a PMD feedforward compensator capable of fully compensating first and second order PMD.
FIG. 2 shows the PMD compensator of the '278 application. PMD detector 25 produces control signals 40, 45, 50 and 55 for PMD compensator 100. The output of PMD compensator 100 is a corrected lightwave signal that is substantially devoid of first and second order PMD. The corrected signal is supplied to receiver 90. PMD detector 25 comprises optical filter 28, polarimeter 30 and processor 35. The operation of PMD detector 25 is generally described in co-pending U.S. patent application Ser. No. 10/263,779, filed Oct. 4, 2002, having a common inventor and assignee herewith, the disclosure of which is hereby incorporated by reference in its entirety. PMD compensator 100 comprises differential group delays (DGDs) 65, 75, 85 serving as three first order PMD segments, and PCs 60, 70, 80, serving as polarization rotators. PCs 60, 70, 80 are each a cascade of two or three tunable wave-plates with fixed slow axis of orientation, or may each be a cascade of two or three fixed wave-plates with adjustable slow axis of orientation. DGD 65 is adjustable and thus enables adjustment of the DGD. Control signals 40, 45, 50 and 55 are respectively supplied to PC 60, DGD 65, PC 70 and PC 80. The '278 application recognizes that the variable DGD segment can be replaced with a concatenation of fixed DGD segments.
Conventionally, a mechanical approach is used to generate variable DGD, that is, the two orthogonal polarization components are separated using a polarization beam splitter and then a path difference is introduced between them. Finally, the two polarization components are recombined using a polarization beam combiner. This approach requires mechanical movements, and thus operates at slow speed (sub-second), and has large output polarization fluctuation and poor control stability. In addition, this mechanical variable delay line introduces substantial optical loss.
Alternatively, variable DGD can be generated by concatenating two fixed DGD segments via a polarization controller. However, this results in a second order PMD vector perpendicular to the resultant first order PMD vector, which causes rotation of the principal state of polarization as one moves away from the center wavelength.
Thus, there is a need for a further improved PMD compensator.
FIG. 3 shows a PMD generator, also referred to as a PMD emulator. In a development lab, or when investigating optical signal behavior, it is desirable to generate PMD and apply it to equipment (not shown) being tested. Generally, PMD controller 7 is programmed to control PMD generator 6 to create desired types of distortion in the lightwave signal from transmitter 10. The intentionally distorted signal is then delivered to receiver 90. Equipment being tested is inserted at an appropriate place.
The operational requirements of PMD generator 6 are less stringent than those of PMD compensator 1, as PMD compensator 1 has to deal with all types of real world conditions in real time, whereas PMD generator 6 is useful even if it operates in only a limited way and relatively slowly.
If the required compensation is within the operational capability of PMD generator 6, then PMD generator 6 can be used in place of PMD compensator 1.
U.S. Patent Application Publication No. 2002/0118455, published Aug. 29, 2002, shows a feedforward PMD generator wherein first order PMD and second order PMD can be selected from separate contours to provide constant DGD.
FIG. 4 shows the PMD generator of the '455 publication. PMD generator 115 comprises four serial stages, with a half-waveplate mode mixer between adjacent stages. Each stage includes a fixed DGD and a phase compensator, shown in FIG. 4 as a phase shifter. Fixed DGDs 101, 104, 107, 110 are substantially identical. Phase compensators 102, 105, 108, 111 are also substantially identical, that is, are controlled to be identical via a control signal from PMD controller 112. Mode mixers 103 and 109 are controlled by the same signal from PMD controller 112. Mode mixer 106 is controlled by its own control signal from PMD controller 112.
By appropriately setting the control signals, PMD controller 112 controls PMD generator 115 to produce a DGD spectrum that is aligned with a wavelength division multiplexed (WDM) comb spectrum, imparting the same amount of PMD to each WDM channel.
The behavior of PMD generator 115 is complicated, and not readily described via equations. The '455 publication teaches creating a set of operational curves for PMD generator 115, apparently by observing its behavior, and then incorporating these operational curves in PMD controller 112 as a control algorithm or look-up table.
Drawbacks of the PMD generator of the '455 publication include that it is slow due to relying on mechanical movement of the phase compensators 102, 105, 108, 111 and mode mixers 103, 106, 109, and that it controls only the magnitude of the generated PMD, i.e., does not control the direction of the generated PMD.
FIG. 5 shows another recent approach to using variable DGD. The device of FIG. 5 is a concatenation of six switch/delay sections. Each switch/delay section consists of a magnetoptic polarization switch and a birefringent crystal. The lengths of the birefringent crystals are a binary power series, increasing by a factor of 2 for each section. While the binary power series lengths approach of FIG. 5 is acceptable for a PMD emulator, it is not suitable for PMD compensation since it does not provide continuous tuning.
A birefringent crystal has different reflective indices for its two different orthogonal polarizations. The fast axis is the one with the smaller reflective index.
The PMD generators described above still leave room for an improved PMD compensator.